In some technical fields, and in particular in aeronautics, safety means must be provided on bolts to prevent the bolt from disengaging from the assembly hole in the event the system of nuts should accidentally become loose, for example due to vibration.
For example, French patent No. 1 506 796 describes a bolt provided at the base of its thread with a groove into which is inserted a cylindrical-base ring which is transversely slotted so that it can elastically deform in the circumferential direction. (Hereafter the expression "transverse" refers to the ring and not to the member supporting it.)
FIG. 1 of the attached drawings illustrates one of the fasteners of this type, which presently is most widely used in aeronautics. FIG. 2 is a cross-sectional detail of this known type of fastener. The ring has a solid cross-section and a cylindrical base and is arranged in the bolt groove so as to project in the rest position from the shank and when in the compressed state, is retracted inside the groove. Such a ring makes it possible to place the bolt in the assembly hole by exerting a penetrating force on the bolt so that, by a radial compression in the hole, the ring will be retracted within the groove. Once the ring has passed through the hole, it returns to rest due to its circumferential elasticity and assures that the bolt shall be kept in the hole.
The ring is shown illustratively in FIGS. 1 and 2 in solid lines in the rest position where it is assumed to be resting on the bottom of the groove by the upper generatrix gs of its cylindrical base (its opposite lower generatrix gi therefore being that which is the farthest from the bottom). The ring in the compressed state is shown in dashed lines in FIG. 2.
To permit complete retraction of the ring within its groove, it is easily seen that the distance "a" separating the generatrix gi from the groove bottom must exceed the height "b" of the ring cross-section. For a given ring, this distance "a" therefore cannot be arbitrarily reduced by the manufacturer.
Under these conditions, it is impossible to reconcile two technical imperatives desired by the aeronautics designers:
on one hand, to achieve as small as possible a groove depth "c" in order not to degrade the bolt strength at that level, this strength being proportional to the square of the diameter of the solid part,
on the other hand to eliminate the danger of tearing off the ring when being inserted into the hole or being removed from it, demanding thereby that the ring side edges dl be housed within the groove over its entire periphery so that said edges shall not hit and hook into the hole edges during this insertion or removal.
As shown in FIG. 2, meeting the former condition implies a shallow groove of depth "c", less than or about the size of "a", whereby the edges dl project from the ring at some part of its periphery, while the latter condition is not met. If on the contrary, the latter condition is met, the bolt groove is deeper in the same bolt, with depth c' shown in dashed lines in FIG. 2, whereby the mechanical strength is degraded at the gorge with respect to the strength of the other shank parts.
The former condition is mandatory for the strength of the assembly, but especially in the case of threaded bolts, breaks appear most frequently near the hole orifice at the base of the thread. That is precisely where the groove is located and any strength reduction at that point degrades in the extreme the strength of the assembly.
The latter condition is important because the danger of tearing off the ring in the first place entails losses in time when the bolt is inserted and furthermore a drop in reliability since a more than trivial percentage of bolts will be without rings following several cycles of use, and this defect may be overlooked in some cases.
Furthermore, it has been noted that the retaining force generated by these known rings strongly decreases with the number of insertions and removals. Thus, a ring of the type shown in FIGS. 1 and 2 generating a retaining force of about 8 kg-f when first used only produces a force of about 2.5 kg-f after 100 tests. This drift represents a serious defect in practice in aeronautics because leading to uncertainty about the actual effectiveness of the emplaced rings.
Moreover, the force of insertion required to put these rings in place for the first uses (about 8 kg-f) as a rule is considered excessive by the aeronautics designers and a force of about 3 to 4 kg-f appears a better compromise, allowing both good retention and satisfactory installation conditions.
Therefore, the primary object of the present invention is to remedy the above drawbacks of the known fasteners.
In particular, one object of the invention is to eliminate the danger of tearing off the ring during the insertion and removal operations without thereby having to deepen the groove housing said ring.
Another object of the invention is to substantially reduce the variations in the retaining force and/or the penetration force during the sequential operations of use.
Another object is to create a fastener which can be adjusted so that the insertion force shall be less than for the known fasteners and in particular approximately between 2 and 4 kg-f.
Another object of the invention is to make possible a significant gain in weight, which is particularly desirable in aeronautics.